We have extracted plaque in order to estimate viable cell mass by adenosine triphosphate (ATP) content as has been used to estimate viable cells in other complex ecologies. The ATP in Actinomyces viscosus, the most difficult of the pure cultures of plaque bacteria to extract, could be released with boiling tris buffer. This extraction method was then used for plaque samples. When DNA was used to determine total cell mass, the ratio of ATP/DNA reflected the fraction of viable cell mass in the sample. In order to analyze very small samples and apply the full potential of the sensitivity of the ATP analysis we will normalize our data on protein instead of DNA. We have adapted the ortho pthalaldehyde analysis for primary amines to an assay for protein sensitive to the lower nanogram range. A combined estimate of viable cell mass by ATP, and total cells in a large number of very small samples with great convenience. This method should be highly applicable to epidemiologic studies and to the study of the effectiveness of antiplaque agents.